For most of the history of the internal combustion engine, mechanical control of valves has been by way of a camshaft. In October 1998, the first electronically-controlled intelligent engine – a MAN B&W 6L60ME – was installed heralding the gradual demise of the camshaft controlled two-stroke. That process is still continuing but now less than one in ten of all two-strokes are camshaft models. The electronic control that replaced the camshaft allows for more flexibility in valve timing permitting improved flexibility in power output and reduced environmental impact.
As can be expected, development of electronic control has not ceased and improvements to valves and openings are regularly made. MAN Energy Solutions has two main types of electronic control in regular use. On the newer ME-C engines the electronic control includes flexible control of fuel injection timing and actuation of exhaust valves, starting valves and cylinder lubrication whereas on earlier type ME-B engines which are still favoured by some owners and remain in production, the injection timing is electronically controlled but actuation of the exhaust valves is camshaft operated, but with electronically controlled variable closing timing.
The development of electronic engine control has allowed the two-stroke to meet the challenges posed by the NOx Code. However, the requirements of Phase III which came into effect in 2016 for new vessels operating ECAs mean other measures are needed as well. Unlike with four-stroke engines, Miller timing is not possible on two strokes so although the electronic control can allow the Tier II requirements to be met quite easily by way of variable exhaust valve closing, two other means are employed on new vessels either alone or in combination depending upon the engine and the operating parameters need to meet the trading strategy of the ship.
Of the other means, Selective Catalytic Reduction (SCR) is a form of exhaust gas cleaning and is carried out after the engine. It can be used on both two-stroke and four-stroke engines. In this system the exhaust gas is directed through a catalytic reactor unit usually with a vanadium catalyst where it meets an injected stream of 40% urea solution. The injection of the reductant can be done in two ways; either airless or air-assisted. High speed engines usually have airless systems while low to medium speed engines use air-assisted injection to dose the exhaust stream. The resultant reaction changes the NOx in the exhaust gas to nitrogen and water.
Applying SCR to two-stroke engines has presented several engineering problems because under normal conditions, the exhaust gas temperature after the turbocharger would be in the 230-260°C range dependent on load and ambient conditions. These low temperatures are problematic for the SCR when HFO is employed. Thus, in order to achieve the highest possible fuel flexibility, it has been necessary to ensure that the engine produces an exhaust gas with the right temperature for the SCR system. The SCR inlet gas temperature should ideally be around 330-350°C when the engine is operated on HFO.
The alternative of Exhaust Gas Recirculation (EGR) is much more suited to NOx reduction on all engine types especially when using low-sulphur fuels. It works by way of recirculating around 40% of the exhaust gas into the engine thus reducing both the temperature and the amount of oxygen in the combustion chamber. Since NOx forms when fuel is burned at high temperatures in air, the system reduces NOx formation. With an EGR system in place there is no need for catalytic reduction.
EGR has been used very successfully in motor vehicles for some time but for two-stroke marine engines it is a relatively new technology and one that is still being developed. One of the challenges for this technology is the positive scavenging differential pressure. For this reason, an EGR blower is necessary to realise exhaust-gas recirculation.
The purpose of the blower is to raise the pressure of the cooled and cleaned exhaust gases so that recirculation through the engine inlet is possible. In this way, a reduction of combustion-temperature peaks – and a subsequent reduction in NOx formation – can be achieved. The required EGR flow varies, depending on load and ambient conditions.
MAN Energy Solutions has developed an electrical turbo blower (ETB) which plays an important role in the operation of the EGR system by providing active speed-control. It is derived from the company’s turbocharger portfolio. The desired EGR operating condition is achieved by using an electrical, high-speed motor directly coupled to the compressor wheel and driven via a frequency converter. A casing unit holds the stator of the motor and provides a supply for cooling water and lube oil for the journal bearings. The interface between the ETB, frequency drive, instruments and control panel in the engine control room is hardwired.
Since May 2015, two ETB18 prototypes have run successfully on an 82,000dwt bulk carrier equipped with a MAN B&W 6S60ME-C 8.2 Tier III engine. The first fully commercial version an ETB40 passed its factory acceptance test in October 2018.
Although EGR can allow two-stroke engines to meet Tier III NOx requirements, there is a problem with it in that the levels of sulphur in fuels means the exhaust gas also contains sulphurous compounds that are corrosive to the engine. To overcome this, the exhaust gas being recirculated passes through a scrubber system to remove some of the corrosive compounds. The wash water from the EGR scrubber needs to be treated before it can be disposed of. At MEPC 73 in October 2018, IMO adopted new guidelines for EGR bleed off water previously agreed at PPR 5. At the time of writing, these had yet to be published.